하수처리 탈리여액과 잉여 슬러지로부터 인 결정화 회수 연구

Abstract

Phosphorus is a substance that is essential for human life, but it is a limited non-renewable resource. Humans can survive without oil, but they cannot survive without water and food from the biological perspective. Phosphorus is one of the three key elements in fertilizers essential for growing food, and it's vital for nurturing crops. The phosphorus humans need is mostly obtained from phosphate rock through industrial processes. However, high-grade phosphate rocks are being gradually depleted, and eventually mining of phosphate is expected to reach its peak around 2040. This cleary indicates that the depletion of phosphorus resources is becoming a new global concern. Therefore, we have now reached a point where we need to deeply consider the various challenges we will face due to the depletion of phosphorus resources. Thus, various studies in this research were conducted to develop and evaluate a phosphorus crystallization and recovery system for obtaining alternative resources of phosphorus rock from sewage treatment plants. This system involves releasing phosphorus from waste activated sludge (WAS) by microwave heating and recovering it through magnesium ammonium phosphate (MAP) crystallization. Valuable research findings could be achieved as follows: Phosphorus recovery from anaerobic digestion centrate was conducted using magnesium chloride, magnesium hydroxide and magnesium oxide under different pH (7.5, 8.0 and 8.5) and Mg/P molar ratio (1.0, 1.5, 2.0, 2.5) conditions. The phosphorus recovery rate increased with increasing pH and Mg/P molar ratio for all magnesium compounds. At pH 7.5, magnesium oxide showed the highest phosphorus recovery rate, followed by magnesium hydroxide and magnesium chloride. However, at a pH of 8.5, a phosphorus recovery rate of more than 90% was obtained for all Mg/P molar ratios. It is expected that magnesium hydroxide and magnesium oxide are able to replace magnesium chloride as a magnesium source in terms of phosphorus recovery efficiency and cost. A MAP crystallization reactor was developed to recover phosphorus from anaerobic digestion centrate and WAS through the analysis of computational fluid dynamics (CFD). First, the reactor with horizontal baffles was developed using CFD, but complete fluid mixing could not be achieved because of the swirl phenomenon (separated into two layers). Therefore, a reactor with both horizontal and vertical baffles was developed to overcome the problem. As a result of installing both vertical baffles, the swirl phenomenon was controlled, making it possible to recover phosphorus as MAP crystals from a single complete stirred tank reactor (CSTR). Moreover, the continuous stirred reactor developed using CFD was installed in Ilsan sewage treatment plan and operated to evaluate the recovery of phosphorous and nitrogen from anaerobic digestion centrate. More than 95% of the phosphate could be recovered from the centrate into MAP crystals. The contents of TP and TN in MAP crystals were 28.1% and 5.17%, respectively. Some heavy metals were identified, but remained under the Korean standards for organic fertilizer. Thus, it is expected that the MAP crystals could be a useful and valuable nutrient fertilizer, which slowly and continuously releases essential nutrients in response to the demand from farming and planting. Microwave heating and conventional heating methods were compared and evaluated to efficiently release phosphorus from WAS. Microwave heating method encouraged more phosphorus release from WAS than conventional heating method. The amount of phosphorus released strongly depended on pH conditions and temperature. Most of phosphorus was released below 59oC within 30 min. It was confirmed that the microwave heating method raises the temperature of WAS faster than the conventional heating method, resulting in the release of more phosphorus . Moreover, the phosphorus crystallization and recovery system was operated for more than 6 months to recover phosphorus as MAP crystals from anaerobic digestion centrate and WAS in the Gwangju sewage treatment plant. The system consisted of microwave heating and MAP crystallization processes. The recovery of phosphorus from WAS was attempted by microwave heating and MAP crystallization. WAS released large amount of phosphate from the cell when it was exposed to high temperature environments. The amount was increased with increasing temperature, showing that 88.4% of the polyphosphate present in the cells were released in the form of phosphate at 70oC. A similar result was also observed in the release of ammonia. Both phosphorus and ammonia were crystallized with magnesium, and then harvested as MAP crystals. The phosphorus recovery rate reached almost 97.7%, but the ammonia recovery rate was about 13.5%. These results clearly shows that phosphorus as MAP crystals could be recovered from WAS. The MAP obtained from the anaerobic digestion centrate was tested to evaluate the fertilizer effect on plant growth. The test was conducted sowing lettuce in raised bed soil with various MAP contents. The number and length of lettuce leaves increased with the increase in MAP content. In addition, both lettuce weight and dried lettuce weight increased. It was confirmed that MAP is suitable for plant growth as slow-release fertilizer which provide phosphorus and other nutrients slowly over time. As shown in the above results, phosphate could be successfully recovered as MAP crystals from centrate and WAS. MAP crystals are an environmentally friendly fertilizer, which slowly releases ammonia and phosphorus in response to the plant root’s demand. Thus, it can be expected that MAP crystals as a phosphate fertilizer will play an important role in food production in the future.